1. Field of the Invention
The invention relates to a heating element device, which is brought into contact with an object to regulate the temperature of the object at a given temperature, a heating element mounted structure for the heating element device, a temperature control circuit for controlling the temperature of the heating element device, a temperature control apparatus including the heating element device and the temperature control circuit, and a module. More particularly, the invention relates to a heating element device, a heating element mounted structure, a temperature control circuit, a temperature control apparatus, and a module which are suitable for low power consumption purposes.
2. Description of the Related Art
Communication networks such as the Internet has explosively spread. This has led to a demand for further increased speed and capacity for backbone transmission systems called “backbone.” DWDM (dense wavelength division multiplexing) for high-density multiplexing of light signals with different wavelengths has drawn attention as means useful for an increase in capacity. In DWDM, a large number of waveguide devices such as AWG (arrayed waveguide) for multiplexing/demultiplexing light are used as devices for processing light signals. Stable operation of these waveguide devices is necessary for realizing dense wavelength division. To this end, the operation temperature should be kept constant. The temperature control of these waveguide devices is important for this.
On the other hand, an attempt to lower the power supply voltage has been made to realize low power consumption. For example, it is common practice to change a power supply voltage of 5 V (volts) to 3.3 V. In the above case, however, lowering the power supply voltage makes it difficult to ensure electric power for keeping the waveguide element constituting the waveguide device at a desired temperature. The reason for this will be explained by taking the voltage of 5 V and the voltage of 3.3 V as an example.
FIG. 1 shows a heater which is subjected to on-off control by a transistor with a power supply voltage of 5 V. The collector-to-emitter voltage Vtr of the transistor 11 of which the collector has been grounded is assumed to be 0.7 V. Further, it is assumed that a 3-Ω resistor 12 is used as a heater between the emitter in the transistor 11 and the power supply. In this case, the voltage Vr across both ends of the resistor 12 is 4.3 V, a value obtained by subtracting the collector-to-emitter voltage Vtr 0.7 V from the power supply voltage 5 V. The resistance value of the resistor 12 is R. The power consumption of the resistor 12 is Pr, and the current flowed through the resistor 12 is IC. At that time, the power consumption Pr5V of the resistor 12 can be calculated by equation (1):                                                                         P                r5V                            =                            ⁢                                                                    I                    c                                    ×                                      V                    r                                                  =                                                      V                    r                    2                                    /                  R                                                                                                        =                            ⁢                                                                    (                                          5                      -                      0.7                                        )                                    2                                /                3                                                                                        =                            ⁢                              6.2                ⁢                                                                   ⁢                W                ⁢                                                                   ⁢                                  (                  watts                  )                                                                                        (        1        )            
On the other hand, FIG. 2 shows a heater which is subjected to on-off control by a transistor with a power supply voltage of 3.3 V. The collector-to-emitter voltage Vtr of the transistor 11 of which the collector has been grounded is likewise 0.7 V because the collector-to-emitter voltage Vtr can be regarded as being substantially kept at a fixed value even when the power supply voltage is varied. Likewise, a 3-Ω resistor 12 is provided as a heater between the emitter in the transistor 11 and the power supply VCC. In this case, the power consumption Pr3.3V of the resistor 12 can be calculated by equation (2):                                                                         P                                  r3                  ⁢                  .3                  ⁢                  V                                            =                            ⁢                                                                    I                    c                                    ×                                      V                    r                                                  =                                                      V                    r                    2                                    /                  R                                                                                                        =                            ⁢                                                                    (                                          3.3                      -                      0.7                                        )                                    2                                /                3                                                                                        =                            ⁢                              2.3                ⁢                                                                   ⁢                W                ⁢                                                                   ⁢                                  (                  watts                  )                                                                                        (        2        )            
In this way, assuming that the collector-to-emitter voltage of the transistor 11 is equal, when the power supply voltage is lowered from 5 V to 3.3 V, the quantity of heat generated from the resistor 12 as the heater is significantly reduced to 37% of the quantity of heat in the case of the power supply voltage 5 V. Therefore, when the power supply voltage is lowered in this way, the same quantity of heat as in the case of the power supply voltage 5 V cannot be generated without the provision of three sets of heaters shown in FIG. 2.
Mere provision of three sets of heaters, however, increases the mount area by at least three times. Therefore, the size of the temperature control apparatus provided with heaters is disadvantageously increased. Apart from this problem, the provision of a plurality of heaters poses an additional problem that the on-off control of the power supply of these heaters disadvantageously lowers thermal efficiency. This problem will be explained by taking the provision of two sets of identical heaters as an example for simplified explanation.
FIG. 3 shows the principal construction of a conventional temperature control apparatus. A temperature control apparatus 21 includes first and second heaters 22, 23 having a resistance of RH which are connected parallel to each other to constitute a parallel circuit. A switching transistor 24 of which the emitter and collector are connected between one end of the parallel circuit and a ground. A heater control circuit 26 is provided for supplying a control signal for switching to the base of the switching transistor 24 and for applying a power supply voltage to a power supply input terminal 25 in one end of the parallel circuit of the first and second heaters 22, 23. In this temperature control apparatus 21, the voltage across both ends of the parallel circuit of the first and second heaters 22, 23 will be hereinafter referred to as heater voltage VH, and the collector-to-emitter voltage of the switching transistor 24 will be hereinafter referred to as switch-side voltage VCE.
In such an initial state that an apparatus (not shown) of which the temperature is to be regulated has not been warmed yet, the temperature control apparatus 21 turns on (energizes) the switching transistor 24 and, in addition, applies the maximum permissible voltage to the power supply input terminal 25. As the temperature of the apparatus of which the temperature is to be regulated rises, the applied voltage is lowered. When the temperature has reached a desired temperature, the applied voltage is varied so as to keep that temperature. As soon as the temperature control of the apparatus has been completed, the switching transistor 24 is turned off (deenergized).
FIG. 4 shows the relationship between the set temperature and the power supply voltage applied to the temperature control apparatus shown in FIG. 3. When the set temperature required of the apparatus of which the temperature is to be regulated is higher, the power supply voltage applied to the power supply input terminal 25 in the temperature control apparatus 21 shown in FIG. 3 is higher.
FIG. 5 shows the relationship between the power supply voltage applied to the temperature control apparatus shown in FIG. 3 and the energy efficiency (the ratio between the power consumption of the heater and the power consumption of the whole temperature control apparatus). As explained above, the switch-side voltage VCE in the switching transistor 24 of the temperature control apparatus 21 is substantially a fixed value independently of the level of the voltage applied to the power supply input terminal 25, for example, about 0.7 V. The voltage obtained by subtracting the switch-side voltage VCE from the power supply voltage is a heater voltage VH. Therefore, when the power supply voltage is low, a relatively larger voltage than the case where the power supply voltage is high is applied to the switching transistor 24 and is wastefully consumed. This means that the thermal efficiency is low.
When the power supply voltage applied to the power supply input terminal 25 is increased, the proportion of the electric power consumed by the switching transistor 24 is relatively reduced. As explained above, however, an attempt to lower the power supply voltage has been made to realize low power consumption. As shown in FIG. 5, when the energy efficiency is lower in a lower power supply voltage region, lowering the voltage does not satisfactorily contribute to a reduction in power consumption.